Processes for the aqueous dispersion polymerization of fluoropolymer have typically employed a fluorosurfactant such perfluorooctanoic acid or its salts as a polymerization agent. Because of environmental concerns with regard to perfluorooctanoic acid and its salts, there is interest in reducing or eliminating perfluorooctanoic acid and its salts in fluoropolymer polymerization processes. Recently, there is an interest in using fluoroether carboxylic acids or salts in place of perfluoroalkane carboxylic acids or salts in the polymerization of fluoropolymers. In addition, fluoropolymer manufacturers desire to capture or otherwise contain the fluoroether carboxylic acids or salts employed. Since aqueous fluoropolymer dispersions made using fluoropolyether acids or salts may contain significant levels of residual fluoroether carboxylic acids or salts, there is a desire to reduce the level of residual fluoroether carboxylic acids or salts in such dispersions.
For example, US 2007/0015864 A1 discloses the use of fluoroether carboxylic acid surfactant in the aqueous dispersion polymerization of fluoropolymers and the reduction the fluoroether carboxylic acid surfactant in the dispersion. The fluoroether carboxylic acid surfactants of US 2007/0015864 A1 have the formula:[Rf—O-L-COO−]iXi+wherein L represents a linear partially or fully fluorinated alkylene group or an aliphatic hydrocarbon group, Rf represents a linear partially or fully fluorinated aliphatic group or a linear partially or fully fluorinated aliphatic group interrupted with one or more oxygen atoms, and Xi+ represents a cation having a valence i and i is 1, 2, or 3. Surfactants of this formula are referred to hereinafter as “linear fluoroether carboxylic acids or salts”.
US 2007/0015864 A1 describes several known techniques as being suitable for the reduction of the linear fluoroether carboxylic acids in the fluoropolymer dispersion: contacting the dispersion with anion exchange resin as disclosed in WO 00/35971; ultrafiltration as disclosed in e.g., U.S. Pat. No. 4,369,266; a method involving decantation as disclosed in EP 1 538 177; and a method involving steam distilling the free acid form of the fluorinated surfactant as disclosed in EP 1 274 771. The fluorosurfactant reduction methods described in US 2007/0015864 A1 are known to be complex and/or have significant disadvantages for the processing of fluoropolymer dispersion.
An improved process for reduction of fluoroether carboxylic acid or salt from fluoropolymer dispersion is desired, especially a simplified process which coordinates well with dispersion concentration operations.